Zoom Background Image in the VSE
It's not possible at the moment. This "VSE Backdrop" feature has been introduced in the gooseberry branch with rB0717d:


  DESCRIPTION
  
  Gooseberry request: Backdrop for sequencer.


There has been another request rB9a934 for an "Overdrop" what is exactly what you saw in your Video which has zoom functionality and can be dragged around. But it differs from the Backdrop feature you asked for. Actualy it's an evolvement of it and got renamed.

But as it is in development with feature branches not everything that is being developed for an Open Movie project finally gets into the Master Branch of Blender. Not every feature is bug free or already polished enough and therefore this would take too much time to merge it back into master.

A detailled description what has been merged back from Gooseberry is in the Blog article Gooseberry in Blender Master and unfortunately for the Sequencer only


  A backdrop feature


has been mentioned there. Not the "Overdrop" feature. So your missing feature has not been removed, it just never made it into the "official" Blender development branch that results into the Version you can download from blender.org/download. You might ask the developers to merge this Overdrop feature back into Master as well.